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1 When the burner of a stovetop is very hot, it is a source of heat energy. 2  Anything placed onto the stovetop and warmed, whether a pot of tea or a skillet for frying eggs, also become sources of heat energy. 3  Automobile fuels such as gasoline are sources of heat energy, as is the hot engine of a racecar or a school bus.
Heat is the thermal energy that is transferred between two systems when at different temperatures. An isolated object may have thermal energy, but there is no heat in the formal sense of the word. When you rub your hands together, you do not generate heat because there is no temperature difference between your hands.